Although a variety of electrical and electronic sensors are known for determining the position of a mechanical element, such devices generally suffer from susceptibility to natural and manmade electromagnetic noise and other environmental effects that can degrade their performance. For this reason, electrically passive optical position sensors offer a clear advantage for use in extreme environments and in applications where very high reliability is important. For example, optical sensors will soon be employed on aircraft for sensing the position of control surfaces and may be incorporated into a servo system in which the position command and the position feedback information are generated by two similar optical sensors.
Either digital or analog encoding techniques can be used in an optical position sensing system to precisely determine the position of a rotary shaft or a linear actuator. In these systems, light signals are usually conveyed to and from the sensors by optical fibers. Typically, light propagating through an optical fiber from a remote source is modulated by an encoded track on a mechanical element that rotates or moves linearly. The modulated light signal is conveyed to a light sensor that determines the position of the mechanical element based on the modulated intensity of the light signal.
Both the reflective and transmissive properties of an encoded track have been used in prior art devices for analog modulation of a light signal to sense position. In the case of transmissive modulation, the density of the encoded track varies as a function of its position relative to the light beam passing through it. Alternatively, the reflectivity of the encoded track can be varied with the position of the moving element relative to the incident light beam so that the intensity of light reflected from the encoded surface determines the surface position. In either case, the optical fiber that conveys the modulated light signal to the light sensor is disposed so that the light modulated by the encoded track is directed into it.
Since the intensity of light reaching the remote light sensor is determinative of the position of the mechanical element, any variation in light intensity not caused by the reflectivity or transmissivity of the encoded track represents an error in this determination. For example, instability in the light source intensity or of the light sensor sensitivity, variable light losses in the optical fiber interconnections, or contamination of the exposed optical surfaces of the position sensor can produce a variation in the light intensity perceived at the light sensor, and thus can contribute a significant error in the position determined by the sensor. Furthermore, any such error occurring after the position sensor is calibrated is not readily detectable.
An analog optical position-indicating sensor is disclosed in U.S. Pat. No. 4,769,537 that attempts to compensate for this type of error. In this sensor, light at three different wavelengths is conveyed through a common optical fiber and directed through an encoded track on a movable element. The encoded track is completely transparent to light at two of the wavelengths, but its transmissivity in respect to the third wavelength varies with the position of the encoded track. Light transmitted through the encoded track is conveyed through another optical fiber to two optical couplers disposed adjacent three light sensors. The optical couplers divide the light into three separate beams, each comprising light at one of the three wavelengths, and direct these beams to the light sensors, which determine the relative intensities of the light at each wavelength. By monitoring the ratio of the various light beam intensities at the light sensors, modulation of light intensity at the third wavelength by the encoded track, and thus the position of the moving element, can be determined independently of spurious variations in light intensity that occur in the system. However, since wavelength discrimination occurs at the encoded track, any variation in its transparency with respect to the two wavelengths that it is not intended to modulate (e.g., due to contamination by dirt) causes an error in the position measured by this device. Because the area of the encoded track is typically relatively small, it is more susceptible to the effects of contamination than a larger area would be. A further disadvantage of this optical position sensor is its relative complexity.
Accordingly, it is an object of the present invention to provide an optical position sensor that compensates for variations in the intensity of light signals propagating through the system so as to minimize their effect on the accuracy of the sensor. It is a further object to provide a relatively compact, low cost optical position sensor that monitors the modulation of a light beam by an encoded surface to determine the position of the surface. A still further object is to reduce the opportunity for contamination of the optical surfaces in such an optical position sensor. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follows.